1. Field of the Invention
This invention relates to a sealing device for a packaging machine for utilizing thermoplastic packaging sheet such as cellophane or polypropylene to form a cover on box-like items such as cigarettes covered by a packaging sheet, for example. More particularly, the invention relates to a sealing device in which annular belts are wound around, in opposition to the right and left sides of the box-like items, along both right and left sides of a transferring passage for continuously transferring the box-like items. For example, both of the annular belts are moved toward a transferring direction of the box-like items in synchronism with their transferring speed, a plurality of heater blocks are disposed within each of the annular belts along the transferring direction, and heat from the heater blocks is transmitted to the folding parts of the folded packaging sheet along the right and left side surfaces of the box-like items through the inner sides of each of the annular belts so as to thermally adhere the folding parts of the packaging sheets.
2. Background and Material Information
In the prior art sealing device for a packaging machine, according to a system disclosed in Japanese Patent Laid-Open No. Hei 1-294406, a plurality of heater blocks are abutted against the inner sides of each of the annular belts and the right and left side surfaces of the box-like items by springs acting with a specified pressure. As the folded box-like items pass between these right and left annular belts, both folded parts are held by the inner side of each of the annular belts, heat from the heater blocks is transmitted to thermally melt both folded parts and at the same time, a plurality of heater blocks are separately heated so that a temperature of each of the annular belts is controlled in response to a transferring speed of the box-like items. Then, a specified amount of heating calories is given to each of the folded parts that is related to a variation in the transfer speed of the box-like items.
In addition, the sealing device for a packaging machine can be constructed as disclosed in Japanese Patent Publication No. Sho 61-39251, for example. A winding device for winding the packaging sheet around the box-like items and a folding device for the projecting ends of the packaging sheet which project from the right and left side surfaces of the wound box-like items in a rectangular cylinder form along the side surfaces are continuously disposed as a pre-stage. An item discharging device for discharging box-like items of poor packaged state quality, a multi-stage piling and discharging device for piling up the items in a predetermined number of stages, and discharging the items into a box packaging stage as a post-stage are continuously disposed and they cooperate with other devices placed thereacross.
However, such a prior art sealing device for a packaging machine has some problems including that a plurality of heater blocks are always abutted against the annular belts. Even if a temperature of the heater blocks is lowered or decreased, for example, the temperature is not decreased rapidly due to its surplus heat. Thus in particular, in case that an operating speed (i.e. a transferring speed of the box-like items) may easily be varied, a total amount of heat transferred to the box-like items in response to the transferring speed may not be changed within a short period of time, even though the transferring speed of the box-like items is required to be rapidly accelerated or rapidly decelerated within a short period of time. As a result, a specified amount of heating calories may not be given to the folded parts of the packaging sheet to be transferred at a different speed, and a finished state of the thermal melting is not made uniform, and a poor item may easily be produced.